1. Field of the Invention
The invention relates to a method of laying foundation material beneath a gravity foundation marine structure placed upon the ocean floor.
2. Description of the Prior Art
Two general methods of laying foundation material under a structure have already been practiced. In both methods, water is used as a carrier fluid to transport sand, the foundation material, to voids beneath the structure. When the suspension consisting of the water and sand reaches the void, the resultant velocity decrease of the suspension causes the sand particles to fall from the flowstream and deposit in the void. The water must leave, or be removed from, the void area to allow more sand and water suspension to reach the void.
In the first known method, the water is drawn from the void by a return pipe. In the second known method, the water dissipates naturally away from the suspension injection point, taking the path of least resistance in exiting the void area.
More specifically, the first method utilizes a suspension supply pipe which extends from each injection point to remote portions of the void. The supply pipe is accompanied by a water return pipe which removes the sand depleted water from the void. The pipes may be moved about within the void to eventually fill all portions of the void. This method requires the installation of an extensive piping header system to supply water to and from the void from each injection point.
In a patentable variation of the above first method in the second method, a single supply of suspension is directed from each supply point, with no return piping to remove surplus water from the void. In U.S. Pat. No. 3,683,632, filed July 28, 1970, Ser. No. 58,771, issued Aug. 15, 1972 to Romke van der Veen et al., this method is used to form a crater around the supply place, the crater subsequently being filled up with foundation material. In operation, suspension is supplied to the crater and the sand initially deposits around the rim of the crater. Additional suspension supplied to the crater causes a portion of the originally deposited rim to collapse outward, which causes a channel to form over the upper edge of the rim. Additional suspension flows through this channel, which eventually seals off from accumulated sand. With this method, the edge of the crater grows gradually outward so as to form a continuous layer of foundation material.
But the proper operation of this method depends upon the free dissipation of water away from the suspension injection point. In other words, this process cannot be used to fill an isolated void. Since there is no path for the water which has carried the sand to the void to leave the void, no further sand carrying water can be supplied to the void. Another disadvantage of this second method, as disclosed in Pat. No. '632, is the required installation of an extensive piping header system to supply sand laden water to each supply point.
In consideration of the size of a typical structure to be placed upon the ocean floor, the magnitude of the required piping system can readily be visualized. For example, a gravity foundation structure for installation in arctic waters may have a lower surface diameter of 550 feet that contacts the ocean floor. Full scale tests have shown that sand can be deposited within a radius of approximately 50' from a single sand and water suspension outlet. The gravity foundation structure would therefore require from 160 to 200 separately piped outlets installed through its lower surface, with associated isolation valves, bulkhead penetrations, and flow directing manifolds. Such a system may only be used several times during the life of the structure, and would require continuous maintenance.
A method needs to be developed that eliminates the disadvantages inherent with the installation and operation of an extensive piping system required for these infrequent jetting operations. This new method should not, however, sacrifice any of the sandjetting capabilities of the two current methods.